In power semiconductor devices, it is required to reduce on-resistances while keeping the high breakdown voltage. In recent years, it is shifted from planar MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) to vertical MOSFETs to meet such requirement. In the vertical MOSFETs, since a channel region is formed in a direction substantially perpendicular to a major surface of a semiconductor substrate, a channel density is increased and reduction in on-resistances can be achieved.
Moreover, to realize still higher breakdown voltage and a further reduction in the on-resistances, three-dimensional semiconductor devices have been considered in which channel regions are provided not only on the major surface but also in a vertical direction of the semiconductor substrate. In the three-dimensional semiconductor device, a source region, a base region and a drain region are provided so as to extend respectively in a direction substantially perpendicular to the major surface of the semiconductor substrate and further a gate electrode in a trench shape is provided. According to the semiconductor device of the foregoing structure, channel regions are formed both in a direction substantially parallel to and in a direction substantially perpendicular to the major surface of the semiconductor substrate. With this structure, a significant improvement in channel density can be realized. As a result, the three-dimensional semiconductor device realizes a reduction in on-resistances while keeping the high breakdown voltage.
However, in the three-dimensional semiconductor device, when the semiconductor device is switched between the on-state and the off-state, a rapid increase in potential difference occurs between the source electrode and the drain electrode, resulting in a temporary overvoltage. For this reason, an avalanche breakdown may occur in the vicinity of the base region and the lower end part of the gate electrode.
Carriers generated in the avalanche breakdown are discharged to the source electrode side via the base region. However, in the three-dimensional semiconductor device, because the base region is formed in depth, carries are likely to stay in the base region. Therefore, a potential in the base region rises, and a bipolar action may be caused by a parasitic bipolar transistor. Furthermore, if this bipolar action occurs in a chain, a device breakdown may occur by a so-called “latch-up”. Therefore, it is desirable to realize a further improvement in the breakdown voltage for the three-dimensional semiconductor device.